Polycarbonate substrates

ABSTRACT

Polycarbonate substrates having low concentration of defects are disclosed. The substrates are suitable for producing molded articles characterized by low cloudiness.

The invention provides polycarbonate substrates and their use forpreparing moulded items with especially high purity and extremely highsurface brilliance and the moulded items which can be prepared from thepolycarbonate substrate.

For certain moulded items such as in particular glazing for motorvehicles and headlamps, glazing of other types such as so-calledtwin-wall sheets or hollow chamber sheets or solid sheets, high puritypolycarbonate is required.

Polycarbonate is prepared by the so-called phase boundary process inwhich dihydroxydiarylalkanes, in the form of their alkali metal salts,are reacted with phosgene in heterogeneous phase in the presence ofinorganic bases such as sodium hydroxide solution and an organic solventin which the product polycarbonate is very soluble. During reaction, theaqueous phase is distributed within the organic phase and after reactionthe organic polycarbonate-containing phase is washed with an aqueousliquid, wherein, inter alia, electrolytes are removed and the washliquid is then separated.

In order to wash the polycarbonate-containing solution, EP-A-264 885suggests stirring up the aqueous wash liquid with the polycarbonatesolution and separating the aqueous phase by centrifuging.

Japanese application JP-A-07 19 67 83 describes a process for preparingpolycarbonate in which the concentration of iron in the sodium hydroxidesolution used is intended to be less than 2 ppm, in order to producefavourable colour characteristics.

The object of the present invention is the provision of an alternativeand improved process for preparing pure polycarbonate substrates and theprovision of polycarbonate moulded items with especially high purity andhigh surface brilliance.

Surprisingly, it has now been found that polycarbonate moulded itemswith especially high purity and high surface brilliance are obtainedfrom polycarbonate substrates which are prepared by a special process.

Therefore, the application provides polycarbonate substrates with a lowconcentration of foreign particles and the polycarbonate moulded itemswith high surface brilliance prepared therefrom which are obtained by aprocess for preparing polycarbonate by the phase boundary method whichis characterised in that, from the group of feedstocks, at least thefeedstock sodium hydroxide solution contains very little Fe, Cr, Ni, Zn,Ca, Mg, Al or their chemical homologues.

The application therefore provides a process for preparing polycarbonateby the phase boundary process, wherein dihydroxydiarylalkanes in theform of their alkali metal salts are reacted with phosgene inheterogeneous phase in the presence of sodium hydroxide solution and anorganic solvent, characterised in that

a) the feedstocks are low in the metals Fe, Cr, Ni, Zn, Ca, Mg, Al ortheir homologues

b) the organic solvent is separated off and

c) the polycarbonate obtained is worked up.

In the context of the invention, low in the metals mentioned or theirchemical homologues means that preferably not more than 2 ppm, morepreferably not more than 1 ppm and particularly preferably not more than0.5 ppm and very particularly preferably not more than 0.2 ppm of totalmetal, in particular of the metals listed above and their homologues iscontained in the feedstocks. These limiting values do not apply to thealkali metals.

It is intended that the feedstock sodium hydroxide solution shouldpreferably be low in the metals mentioned. In particular, with referenceto a 100 wt. % strength NaOH concentration, the sodium hydroxidesolution should contain not more than 1 ppm, preferably not more than0.5 ppm, more preferably not more than 0.3 ppm of alkaline earth metalsor their homologues. In particular, with reference to a 100 wt. %strength NaOH concentration, the feedstock sodium hydroxide solutionshould contain not more than 1 ppm, advantageously not more than 0.5ppm, preferably not more than 0.1 ppm of iron.

The sodium hydroxide solution is preferably used in the processaccording to the invention as a 20-55 wt. % strength, particularlypreferably a 30-50 wt. % strength solution.

Sodium hydroxide solution with the limiting values mentioned above isobtainable by membrane processes known from the literature.

In a preferred embodiment, in addition to the sodium hydroxide solution,the feedstocks bisphenol, in particular bisphenol and water, veryparticularly preferably bisphenol, water and the organic solvent are lowin metals, in particular low in Fe, Cr, Ni, Zn, Ca, Mg, Al or theirchemical homologues.

Embodiments in which a sodium bisphenolate (solution) has beenpreviously prepared from sodium hydroxide solution and bisphenol(s) arealso included.

These feedstocks which are low in metal are obtained, in a preferredvariant, by distilling the solvent, crystallising the bisphenol,preferably crystallising or distilling the bisphenol several times andusing the water in a fully deionised quality.

The fully deionised water is preferably desalted, degassed and/ordesilicified. The electrical conductivity (sum parameter for ionogenicsubstances from salts which are still present in trace amounts in thewater) is used as a quality criterion, wherein in the process accordingto the invention the fully deionised water is characterised by anelectrical conductivity of 0.2 μS/cm (DIN 38404 C 8) and a SiO₂concentration of 0.02 mg/kg (VGB 3.3.1.1), or less than each of these.

The concentration of dissolved oxygen in the fully deionised water isadvantageously less than 1 ppm, preferably less than 100 ppb. Thisoxygen concentration is preferably applied to all starting substancesand process steps.

In a further preferred embodiment, from among the group of feedstocks,at least the sodium hydroxide solution, preferably also the bisphenol,particularly preferably the sodium hydroxide solution, the bisphenol andthe water, most preferably the sodium hydroxide solution, the bisphenol,the water and the organic solvent are filtered once, preferably twice,particularly preferably three times step-wise, before the start ofreaction.

The invention also provides a process for preparing polycarbonate by thephase boundary process, wherein dihydroxydiarylalkanes in the form oftheir alkali metal salts are reacted with phosgene in heterogeneousphase in the presence of sodium hydroxide solution and an organicsolvent, characterised in that

a) the feedstocks are low in the metals Fe, Cr, Ni, Zn, Ca, Mg, Al ortheir homologues

b) the aqueous phase being produced during reaction is separated and theseparated organic polycarbonate phase is washed with an aqueous liquidand

c) the washed, and separated from the wash liquid, organic polycarbonatephase, optionally after filtration, is heated and filtered hot at leastonce;

d) the organic solvent is separated off and

e) the polycarbonate obtained is worked up.

In a preferred embodiment, in process step d), the reaction mixture isfiltered directly after reaction and/or the organic polycarbonate phaseobtained and separated is filtered and/or the organic polycarbonatephase separated in process step e) is filtered.

Preferably at least two of these filtration procedures, in particularall three filtration procedures, are performed.

In a preferred variant, in particular in the case of hot filtration, themixture is filtered at least once, preferably twice, particularlypreferably at least three times, in particular step-wise. In the case ofstep-wise filtration, coarser filters are used first and then these arereplaced by finer filters. The filtration procedure for the two-phasemedia in process step d) is preferably performed with coarse filters.

In process step e), filters with smaller pore sizes are used for hotfiltration. In this case it is important that the polycarbonate phase ispresent as the most homogeneous solution possible. This is achieved byheating the organic polycarbonate phase, which generally still containsresidues of aqueous wash liquid. The wash liquid then dissolves and aclear solution is produced. The previously dissolved contaminants, inparticular dissolved alkali metal salts, precipitate out and can befiltered off

In order to achieve a homogeneous solution, the well-known freezing outmethod may be used in addition to the method described above.

To perform filtration in accordance with the invention, membrane filtersand sintered metal filters or also bag filters may be used as filters.The pore size of the filters is generally 0.01 to 5 μm, preferably 0.02to 1.5 μm, more preferably 0.05 to 1.0 μm. These types of filters arecommercially available, for example from Pall GmbH, D-63363 Dreieich,and Krebsböge GmbH, D-42477 Radevormwald (SIKA-R CU1AS type).

Greatly improved filter lifetimes are obtained by the combination in theprocess according to the invention.

Performing the other process steps is generally well-known. Thus, theaqueous phase is emulsified in the organic phase during reaction.Droplets of differing sizes are then produced. After reaction, theorganic polycarbonate-containing phase is normally washed several timeswith an aqueous liquid and separated as far as possible from the aqueousphase after each wash process. Washing is preferably performed with veryfinely filtered water which has a very low metal content. The polymersolution is normally cloudy after washing and separation of the washliquid. The wash liquids used are aqueous liquids, a dilute inorganicacid such as HCl or H₃PO₄ to separate the catalyst, and fully deionisedwater for further purification. The concentration of HCl or H₃PO₄ in thewash liquid may be, for example, 0.5 to 1.0 wt. %. The organic phase iswashed, for example and preferably, five times.

Well-recognised separating funnels, phase separators, centrifuges orcoalescers or also combinations of these devices may be used as phaseseparation devices to separate the wash liquid from the organic phase.

The solvent is evaporated off to obtain the high purity polycarbonate.Evaporation may take place in several evaporation stages. In accordancewith a further preferred embodiment of this invention, the solvent orsome of the solvent is removed by spray drying. The high puritypolycarbonate is then obtained as a powder. The same applies torecovering the high purity polycarbonate by precipitation from theorganic solution and then removing residues by drying. Extrusion, forexample, is a suitable means of evaporating residual solvent.Extrusion-evaporation technology is another means.

Compounds which are preferably used as feedstocks are bisphenols of thegeneral formula HO-Z-OH, in which Z is an organic group with 6 to 30carbon atoms, which contain one or more aromatic groups. Examples ofsuch compounds are bisphenols which belong to the group ofdihydroxyphenyls, bis(hydroxyphenyl)alkanes, indanebisphenols,bis(hydroxyphenyl)ethers, bis(hydroxyphenyl)sulfones,bis(hydroxyphenyl)ketones andα,α′-bis(hydroxyphenyl)diisopropylbenzenes.

Particularly preferred bisphenols which belong to the previouslymentioned group of compounds are 2,2-bis-(4-hydroxyphenyl)-propane(BPA/bisphenol-A), tetraalkyl-bisphenol-A,4,4-(meta-phenylenediisopropyl)-diphenol (bisphenol M),1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethylcyclohexanone and optionallymixtures of these. Particularly preferred copolycarbonates are thosebased on monomeric bisphenol-A and1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane. The bisphenolcompounds to be used according to the invention are reacted withcarbonic acid compounds, in particular phosgene.

Polyestercarbonates are obtained by reaction of the bisphenols alreadymentioned, at least one aromatic dicarboxylic acid and optionally carbondioxide. Suitable aromatic dicarboxylic acids are, for example,orthophthalic acid, terephthalic acid, isophthalic acid, 3,3′- or4,4′-diphenyldicarboxyilic acid and benzophenonedicarboxylic acid.

Inert organic solvents used in the process are, for example,dichloromethane, the various dichloroethanes and chloropropanecompounds, chlorobenzene and chlorotoluene. Dichloromethane and mixturesof dichloromethane and chlorobenzene are preferably used.

The reaction can be accelerated by catalysts such as tertiary amines,N-alkylpiperidines or onium salts. Tributylamine, triethylamine andN-ethylpiperidine are preferably used. A monofunctional phenol such asphenol, cumylphenol, p-tert.-butylphenol or4-(1,1,3,3-tetramethylbutyl)phenol may be used as chain terminators andmolecular weight regulators. Isatin-biscresol, for example, may be usedas a branching agent.

To prepare high purity polycarbonates, the bisphenols are dissolved inan aqueous alkaline phase, preferably sodium hydroxide solution. Thechain terminators optionally required for preparing copolycarbonates aredissolved in the aqueous alkaline phase or added to this as such in aninert organic phase in amounts of 1.0 to 20.0 mol % per mole ofbisphenol. Then phosgene is passed into the mixer which contains theremainder of the reaction constituents and polymerisation is performed.

Optionally used chain terminators are either monophenols ormonocarboxylic acids. Suitable monophenols are phenol itself, alkylphenols such as cresols, p-tert.-butylphenol, p-cumylphenol,p-n-octylphenol, p-iso-octylphenol, p-n-nonylphenol andp-iso-nonylphenol, halophenols such as p-chlorophenol,2,4-dichlorophenol, p-bromophenol and 2,4,6-tribromophenol and mixturesof these.

Suitable monocarboxylic acids are benzoic acid, alkylbenzoic acids andhalobenzoic acids.

Preferred chain terminators are phenols of the formula (I)

in which

R is hydrogen, tert.-butyl or a branched or unbranched C₈ and/or C₉alkyl group.

Preferred chain terminators are phenol and p-tert.-butylphenol.

The amount of chain terminator to be used is 0.1 mol % to 5 mol %, withrespect to moles of each of the diphenols used. The addition of a chainterminator may take place before, during or after phosgenation.

Optionally, a branching agent may also be added to the reaction.Preferred branching agents are the trifunctional or more thantrifunctional compounds known from polycarbonate chemistry, inparticular those with three or more than three phenolic OH groups.

Branching agents are also, for example and preferably, phloroglucine,4,6-dimethyl-2,4,6-tri-(4-hydroxyphenyl)-hept-2-ene,4,6-dimethyl-2,4,6-tri-(4-hydroxyphenyl)-heptane,1,3,5-tri-(4-hydroxyphenyl)-benzene, 1,1,1-tri-(4-hydroxyphenyl)ethane,tri-(4-hydroxyphenyl)-phenylmethane,2,2-bis-[4,4-bis-(4-hydroxyphenyl)-cyclohexyl]-propane,2,4-bis-(4-hydroxyphenyl-isopropyl)-phenol,2,6-bis-(2-hydroxy-5′-methylbenzyl)-4-methylphenol,2-(4-hydroxyphenyl)-2-(2,4-dihydroxyphenyl)-propane, esters ofhexa-(4-(4-hydroxyphenyl-isopropyl)-phenyl)-orthoterephthalic acid,tetra-4-hydroxyphenyl)-methane,tetra-(4-(4-hydroxyphenyl-isopropyl)-phenoxy)-methane and1,4-bis-(4′,4″-dihydroxytriphenyl)-methyl)-benzene and also2,4-dihydroxybenzoic acid, trimesic acid, cyanuric chloride and3,3-bis-(3-methyl-4-hydroxyphenyl)-2-oxo-2,3-dihydroindole.

The amount of optionally used branching agent is 0.05 mol % to 2 mol %,again with respect to moles of each of the diphenols used.

The branching agent may be either initially introduced in the aqueousalkaline phase with the diphenols and the chain terminators, or addedbefore phosgenation dissolved in an organic solvent.

Some, up to 80 mol %, preferably 20 to 50 mol %, of the carbonate groupsin the polycarbonates may be replaced by aromatic dicarboxylic estergroups.

Polycarbonates according to the invention are either homopolycarbonatesor copolycarbonates and mixtures of these. Polycarbonates according tothe invention may be aromatic polyestercarbonates or polycarbonateswhich are present in a mixture with aromatic polyestercarbonates. Theexpression polycarbonate is regarded as representing the polycarbonatesubstrate obtainable by the process according to the invention.

Polycarbonates according to the invention have average molecular weightsM_(w) (determined by measuring the relative viscosity at 25° C. indichloromethane and at a concentration of 0.5 g of polycarbonate per 100ml of dichloromethane) of 12,000 to 400,000, preferably 23,000 to 80,000and in particular 24,000 to 40,000.

Moulded items according to the invention which can be made from the highpurity polycarbonate according to the invention are in particularglazing for motor vehicles and headlamps, glazing of other types such asfor greenhouses, so-called twin-walled sheets or hollow chamber sheetsor solid sheets. These moulded items are prepared by known methods suchas e.g. injection moulding, extrusion and extrusion blow mouldingprocesses using polycarbonates according to the invention.

Moulded items produced from polycarbonates according to the inventionhave a particularly low number, fewer than 250, preferably fewer than150, of defects per m² measured on a 200 μm extruded film.

The advantage of moulded items according to the invention, in particularfor solid sheets and twin-walled sheets, is their extremely low numberof optically detectable surface defects and their outstandingbrilliance.

Another advantage of moulded items according to the invention, in forparticular discs and headlamp diffusers, is the low cloudiness of lessthan 0.5%, in particular less than 0.4%, associated with long-termdurability.

The following examples are used to explain the invention. The inventionis not restricted to these examples.

EXAMPLES Example 1

To prepare the polycarbonate, BPA (BPA, as a melt, is continuouslybrought into contact with sodium hydroxide solution) is mixed withsodium hydroxide solution with the exclusion of oxygen. The sodiumhydroxide solution used has different concentrations and purities (seetable 1), wherein the original sodium hydroxide solution is dilutedstill further to a 6.5% strength sodium hydroxide solution, usingfiltered fully deionised water, in order to dissolve the bisphenols.This sodium bisphenolate solution is now filtered (0.6 μa filter) andused in the polycarbonate reaction. After reaction, the reactionsolution is filtered through a 1.0 μnom bag filter and taken to a washprocedure. Here, the mixture is washed with 0.6% strength hydrochloricacid and then post-washed another S times using filtered fully deionisedwater. The organic solution is separated from the aqueous solution and,after heating the organic solution to 55° C., it is filtered first witha 0.6 μa filter and then through a 0.2 μa filter. After isolation,poly-2,2-bis-(4-hydroxyphenyl)-propane-carbonate is obtained. Thepolycarbonate obtained has an average molecular weight M_(w) of 31,000.

TABLE 1 Quality of sodium hydroxide solution 1 2 3 % NaOH 50 50 32 Fe(ppm) 0.7 0.46 0.02 Ca (ppm) 2.0 0.4 <0.1 Mg (ppm) 0.5 0.2 <0.1 Ni (ppm)0.2 0.2 <0.01 Cr (ppm) 0.4 0.25 <0.01 Zn (ppm) 0.1 0.05 0.06 Total (ppm)3.9 1.56 <0.3 Conc. in 100% NaOH Fe (ppm) 1.4 0.9 0.06 Ca (ppm) 4.0 0.8<0.3 Mg (ppm) 1.0 0.4 <0.3 Ni (ppm) 0.4 0.4 <0.03 Cr (ppm) 0.8 0.5 <0.03Zn (ppm) 0.2 0.1 0.19 Total (ppm) 7.8 3.1 <0.9

The filter lifetimes for various filter locations are, for individualcases:

TABLE 2 Sodium hydroxide prepared from trial no. Filter lifetimes 1 2 3Before reaction 12 h 10 d 30 d 0.6 μa filter After reaction 24 h 30d >60 d   1.0 μa filter Final filter 1 = 0.6 μa filter 12 h  3 d 21 dFinal filter 2 = 0.2 μa filter

Films are now extruded from the polycarbonates prepared with sodiumhydroxide solution from trials 1 to 3 and these are subjected to a filmlaser scan test using the known method which is described below.

The extruded film is 200 μm thick and 60 mm wide. A He/Ne laser (“spotdiameter” of 0.1 mm) scans the film with a scan frequency of 5000 Hzacross the width and with a rate of transport of 5 m/s in thelongitudinal direction. All defects which cause scattering of thetransmitted laser beam (from 0.10 mm diameter upwards) are detected by aphotomultiplier and counted using a software package. The number ofoptical defects per kg of polycarbonate or per m²of film is a measure ofthe surface quality of these films and the purity of the PCs.

Evaluating the extruded film with a laser scanner PC prepared usingsodium hydroxide soln. from trial no. # per m² of surface 1 2 30.10-0.30 mm 148 93 35 >0.30 mm 232 132 41 Total 380 225 76

Colour sample platelets are produced from the polycarbonates ofdifferent qualities. The colour sample platelets had, according to ASTMD1003, different cloudiness values, wherein the colour sample platelet(4 mm thick) which had been produced with polycarbonates prepared fromsodium hydroxide solution from trial 3 had a particularly low cloudinessof 0.38%, which leads to less diffuse light scattering when used asglazing/headlamp diffusers.

What is claimed is:
 1. Polycarbonate substrates with a concentration ofoptically detectable surface defects that is fewer than 250 defects perm², measured on an extruded film of 200 μm in thickness. 2.Polycarbonate substrates with a concentration of optically detectablesurface defects that is fewer than 150 defects per m², measured on anextruded film of 200 μm in thickness.
 3. A polycarbonate substratehaving a concentration of optically detectable surface defects lowerthan 250 defects per m², as measured on an extruded film of 200 μm inthickness, the polycarbonate having being prepared by the phase boundaryprocess wherein feedstocks are characterized by their low concentrationof FE, Cr, Ni, Zn, Ca, Mg, Al and their homologues.
 4. The substrate ofclaim 3 wherein the concentration of optically detectable surfacedefects is lower than 150 defects per m², as measured on an extrudedfilm of 200 μm in thickness.